If you check PS / Prefs / Camera Raw you can see a couple settings for when to use the ACR plug-in for JPGs and TIFs. I think the options Never, Always, and With-Settings. You can obviously change this preference to Always and open your JPG into Camera Raw that way. Another way is to right-click on a JPG in Bridge and choose Open in Camera Raw. Finally, at least on Windows, there is a way using File / Open As, to set the format to Camera Raw instead of JPG and it works. I think the Mac is a little different but I think you can still use Open to set Camera Raw as the format to use.
I usually open them from Bridge, using Ctrl-R.
Thank you both very much for your kind assistance. I went to PS . Pref > Camera Raw and found that it was already set to "with settings." What does "with settings" mean - or, more importantly, how does that enable me to open jpegs in ACR? When I right click on a jpeg one of the items listed is "open with," but ACR is not one of the options. I tried "other" and still found no way to get to ACR.
Thanks to your suggestion, I've discovered that I can open jpegs in ACR if I use Bridge. I had stopped using Bridge when I got my Mac and discovered that Finder is several time faster than Bridge when searching for images either by name or keywords. But for those instances where I only have jpeg captures, I can certainly use Bridge.
Again, many, many thanks for your help.
On the mac version using File>Open in photoshop, select the jpeg file, set Enable to All Readable Documents and set the Format to Camera Raw
In the camera raw preferences: Automatically open jpegs With Settings means all jpegs that have opened in camera raw before
Automatically open All supported jpegs means all jpegs will open into camera raw